Process for the preparation of asbestos-free microporous electroconductive substrate

ABSTRACT

Process for the preparation of an asbestos-free microporous electroconductive substrate. The process includes (a) providing an aqueous suspension of a mixture of carbon or graphite fibers, polytetrafluoroethylene fibers, inert mineral fibers, at least one fluorinated polymer, a silica porogen, and, optionally, at least one thickening agent; (b) depositing a coating onto a porous support by programmedly vacuum filtering said suspension therethrough, the coating comprising a liquid medium; (c) removing the liquid medium from said coating and then drying same; (d) sintering the coating thus formed; and (e) extracting the silica porogen therefrom. A substrate including intimate admixture of carbon or graphite fibers, polytetrafluoroethylene fibers and inert mineral fibers, and optionally, at least one thickening agent, consolidated by a binding amount of at least one fluorinated polymer is formed. The substrate produced is well suited for the electrolysis of solutions of, e.g., alkali metal halides.

This application is a divisional of application Ser. No. 08/265,786,filed Jun. 27, 1994, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improved microporous electroconductingmaterials and cathodes/electrolytic cells comprised thereof, inparticular cathodes/electrolytic cells for the electrolysis of alkalimetal halide solutions.

This invention also relates to a process for the preparation of suchnovel electroconductive substrates.

The present invention further relates to composites of such cathodesthat include a diaphragm and the electrolysis of alkali metal halidesolutions utilizing same.

2. Description of the Prior Art

Chlorine and sodium hydroxide (caustic soda) are conventionally producedvia electrolysis of aqueous solutions of sodium chloride (chlor-alkaliunits).

Materials suitable for use as cathodes in such NaCl electrolytic cellsmust possess certain properties, i.e., low electrical resistivitycompatible with conducting the electrolysis at an acceptable energylevel, small thickness (about 0.1 to 10 mm), and a large surface areawhich can be up to several square meters.

In addition, it must be possible to produce these materials bydeposition onto a rigid structure having a great number of largediameter openings.

Such electroconductive materials are typically produced by vacuumfiltration of a suspension of fibers and binders therefor.

The properties of the material depend on a number of parameters, inparticular the nature and concentration of the fibrous matter insuspension, surfactants, porogens and other additives.

A fibrous material is known to this art which combines such propertiesand which comprises a mixture of conducting and nonconducting fibers, asdescribed in EP-A-0,319,517. This material comprises a mixture ofasbestos and carbon fibers, with the carbon fibers imparting theelectroconductivity, and the asbestos fibers fixing the binders duringfiltration.

Also, a number of improvements have been made to this material and tothe preparative technique for the manufacture thereof.

Thus, EP-A-0,214,066 describes materials comprising carbon fibers havinga monodisperse distribution of lengths and exhibiting considerablyimproved properties and quality, namely, a far betterperformance/thickness ratio.

EP-A-0,296,076 describes an electroactive material comprising anelectrocatalytic agent which is uniformly distributed throughout itsmass, said agent being selected from among Raney metals and Raney alloysand from which a major portion of the readily removable metal(s) hasindeed been eliminated.

All of the cathode elements described ensure considerable currentdistribution and are suited for use in electrolytic cells that alsocomprise a membrane or diaphragm separating the anode and cathodecompartments.

Nonetheless, the quality of the aforedescribed cathode elements, as wellas others known to this art, is not completely satisfactory because ofthe requirement for asbestos fibers. In addition to the risks to humanhealth attributed to the handling of this dangerous substance, a numberof problems are associated with the inherent chemical instability ofasbestos, such as too short a useful life of electrolytic cellscontaining such cathodes and difficulties in modifying the operatingconditions of electrolysis, for example when increasing the electricpower and/or the concentration of alkali metal hydroxide.

EP-A-0,222,671, assigned to the assignee hereof, describes, in oneexample, production of an asbestos-free precathode from a suspensionwhich contains only conducting carbon fibers. The composition of thesuspension, however, does not permit an industrial coating to beproduced having a porosity which is both fine and regular and whichfirmly adheres to the cathode.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofnovel asbestos-free electroconductive substrates and cathodes shapedtherefrom that are well suited for the electrolysis of alkali metalhalides and which avoid or conspicuously ameliorate the abovedisadvantages and drawbacks to date characterizing the state of thisart.

Briefly, the present invention features novel microporouselectroconducting substrates comprising a fibrous asbestos-free coating,said coating containing carbon or graphite conducting fibers,polytetrafluoroethylene fibers, inert mineral fibers, at least onefluorinated polymer binder for the fibers and, if necessary, at leastone thickening agent.

This invention also features cathodic elements comprising such novelmaterials/substrates.

Also featured hereby is a process for the preparation of said novelmicroporous electroconducting material comprising:

(a) preparing an aqueous suspension having the following approximatecomposition:

(i) 100 parts dry weight of a mixture of fibers which comprises 20 to 80parts dry weight of carbon or graphite fibers and 80 to 20 parts dryweight of polytetrafluoroethylene fibers;

(ii) 10 to 100 parts dry weight of inert mineral fibers;

(iii) 10 to 60 parts dry weight of a fluorinated polymer binder for saidfibers;

(iv) 30 to 200, preferably 30 to 100 parts dry weight of silica basedderivatives; and

(v) 0 to 30 parts dry weight of at least one thickening agent;

(b) depositing a coating onto a porous support by programmed vacuumfiltration of said suspension therethrough;

(c) draining the liquid medium therefrom and drying the coating thusobtained;

(d) sintering the coating; and

(e) removing the silica based derivatives therefrom.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THEINVENTION

More particularly according to the present invention, by "microporouselectroconducting material", it is intended a microporous material orsubstrate having an electrical resistivity ranging from 0.5 to 15 Ω.cm.For reasons of energy consumption, this resistivity preferably rangesfrom 0.5 to 10 Ω.cm, and more preferably from 0.5 to 2 Ω.cm.

In a preferred embodiment of the invention, the mixture of carbon orgraphite fibers and polytetrafluoroethylene fibers comprises 60 to 80parts by dry weight of the carbon or graphite fibers and 40 to 20 partsby dry weight of the polytetrafluoroethylene fibers.

Advantageously, the amounts of inert mineral (or inorganic) fibers and,if necessary, of thickening agent, employed during the subject processand present in the fibrous coating in accordance with the presentinvention, respectively range from 20 to 60 parts by dry weight and from0 to 10 parts by dry weight.

In actual practice, the aqueous suspension prepared in step (a) of theprocess of this invention is advantageously a suspension containingabout 2% to 5% of dry solids.

The electroconductive materials/substrates according to the invention,cathodes comprised thereof, whether simple or composite, and the use ofsame in electrolytic processing, will now be more fully described.

The polytetrafluoroethylene fibers, hereinafter designated PTFE fibers,according to the present invention may have variable dimensions. Thediameter (D) generally ranges from 10 to 500 μm and the length (L) issuch that the ratio L/D ranges from 5 to 500. Preferably, the PTFEfibers have average dimensions ranging from I to 10 mm in length andfrom 50 to 200 μm in diameter. Their preparation is described in U.S.Pat. No. 4,444,640. This type of PTFE fiber is well known to this art.

The carbon or graphite fibers are in the form of filaments having adiameter generally less than 1 mm, preferably ranging from 10⁻⁵ to 0.1mm, and a length greater than 0.5 mm, preferably ranging from 1 to 20mm.

These carbon or graphite fibers preferably have a monodispersed lengthdistribution, i.e., a distribution of lengths such that the length of atleast 80%, and advantageously at least 90%, of the fibers correspond tothe average length ±20%, preferably ±10%.

By the term "inert mineral (or inorganic) fibers", it is intended to beany mineral fiber which is chemically inert in respect of the productsformed during electrolysis in an electrolytic cell comprising same. Thefibers are used to consolidate or reinforce the diaphragm, withoutadversely affecting the wettablity and conductivity of the composite.The mineral fibers must, therefore, be inert in respect to the causticsoda formed when the electrolyte comprises sodium chloride.

Preferred fibers are titanate fibers, calcium sulfoaluminate (ettringit)fibers, ceramic fibers (such as zirconium dioxide, silicon carbide andboron nitride fibers), titanium oxide fibers having the general formulaTi_(n) O_(2n-1) where n is a whole number ranging from 4 to 10 (forexample Ebonex™ marketed by ICI), whether used alone or in admixture.More preferably, titanate fibers are employed.

The titanate fibers are known materials; potassium titanate fibers arecommercially available. Other such fibers are described inFR-A-2,555,207 and are derived from potassium octatitanate K₂ Ti₈ O₁₇ bypartial replacement of the titanium ions in the oxidation state of IV bymagnesium and nickel cations, or in the oxidation state of III such asiron or chromium cations, with charge compensation remaining assured byalkali metal ions such as sodium and potassium cations.

Other titanate fibers such as potassium tetratitanate (K₂ Ti₄ O₉) orderivatives thereof may also be used.

Compounds based on cellulose fibers which have been positively ionicallycharged may also be used. Such compounds are marketed by Beco under thetrademark Becofloc®. These are used in amounts of from 0 to 100 partsdry weight.

By the term "thickening agent", it is intended to be a compound whichincreases the viscosity of the solution and exhibits water-retainingproperties. The natural or synthetic polysaccharides are exemplarythereof. Biopolymers obtained by fermentation of a hydrocarbon in thepresence of microorganisms are particularly preferred. Xanthan gum isespecially representative. The xanthan gum is synthesized using bacteriaof the genus Xanthomonas, more particularly the species described inBergey's Manual of Determinative Bacteriology (8th edition, 1974,Williams N. Wilkins Co., Baltimore) such as Xanthomonas begoniae,Xanthomonas campestris, Xanthomonas carotae, Xanthomonas hederae,Xanthomonas incanae, Xanthomonas malvacearum, Xanthomonas papavericola,Xanthomonas phaseoli, Xanthomonas pisi, Xanthomonas vasculorum,Xanthomonas vesicatoria, Xanthomonas vitians, Xanthomonas pelargonii.Xanthomonas campestris is particularly suitable for the synthesis ofxanthan gum.

Other microorganisms which produce polysaccharides having similarproperties are bacteria of the genus Arthrobacter, Erwinia, Azobacter,Agrobacter or fungi of the genus Sclerotium.

The xanthan gum may be produced via any known technique. Thepolysaccharide is conventionally isolated from the fermentation must byevaporation, drying and milling, or by precipitation by means of a loweralcohol, separation from the liquid, drying and milling, to obtain apowder. Commercially available powders normally have a granulometryranging from 50 to 250 μm and a bulk density greater than about 0.7.

Fluorinated polymers are used as the consolidating binder for theelectroconducting materials in accordance with the invention.

By the term "fluorinated polymers", it is intended to be homopolymers orcopolymers derived at least in part from olefinic monomers substitutedby fluorine atoms, or substituted by a combination of fluorine atoms andat least one chlorine, bromine or iodine atom per monomer.

Exemplary fluorinated homopolymers and copolymers are polymers andcopolymers prepared from tetrafluoroethylene, hexafluoropropylene,chlorotrifluoroethylene and bromotrifluoroethylene.

These polymers may also comprise up to 75 mole % of other derivatives ofethylenically unsaturated comonomers containing at least as manyfluorine atoms as carbon atoms, for example vinylidene (di) fluoride andvinyl or perfluoroalkyl esters such as perfluoroalkoxyethylene.

The fluorinated polymer is advantageously employed in the form of anaqueous dispersion typically containing 30% to 70% of dry polymer havinga granulometry of from 0.1 to 5 micrometers, preferably from 0.1 to 1micrometer.

Polytetrafluoroethylene is the preferred fluorinated polymer.

By the term "silica based derivatives", it is intended to beprecipitated silica and combustion or pyrogenic silica.

The silica advantageously has a BET specific surface area ranging from100 m² /g to 300 m² /g and/or a granulometry, measured using a Coulter®meter, of from 1 to 50 μm, preferably from 1 to 15 μm.

These derivatives are excellent porogens which exert essentially nodeconsolidation influence on the microporous electroconducting materialwhen used in the amounts employed in the present invention. Thesederivatives also serve as network-forming agents for the latexconstituting the binder.

Elimination or removal of the silica based derivatives may be carriedout by alkaline attack. This extraction creates the microporosity in thematerial of the present invention. The silica based derivatives may beeliminated before the microporous electroconducting material is used,but it is more practical and advantageous to remove the silica basedderivatives "in situ" in the electrolytic cell by dissolving it withalkaline medium, particularly during the first few hours ofelectrolysis. Extraction is thus advantageously carried out viacontacting with an aqueous sodium hydroxide solution at a concentrationof between 40 and 200 g/l and a temperature of from 20° to 95° C.

During preparation of the suspension in step (a) of the presentinvention, it is preferable to add at least one surfactant. The maximumamount of surfactant present is typically 10 parts dry weight, andpreferably the amount of surfactant ranges from 0.5 to 5 parts dryweight. A nonionic surfactant is preferably used, in particularethoxylated alcohols or fluorocarbon compounds containing functionalgroups, whether used alone or in admixture. The alcohols or fluorocarboncompounds typically have C₆ to C₂₀ carbon atom chains. Ethoxylatedalcohols which are ethoxylated alkylphenols, in particular octoxynols,are the preferred.

Other elements may be added to the suspension to step (a) of the processof the invention. These additives may, in particular, beelectrocatalytic agents selected from among Raney metals and alloys andmixtures thereof, from which the major fraction of the readily removablemetal(s) are eliminated. Electroactive materials comprising anelectrocatalytic agent of this type are described in EP-A-0,296,076,assigned to the assignee hereof.

In accordance with the process of the present invention, the coating isformed/deposited by programmed vacuum filtration of said suspensionthrough a porous support. These porous supports may, in particular,comprise a gauze or screen having a mesh size, perforations, orporosity, ranging from between 20 μm and 5 mm. These porous supports mayhave one or more planar or cylindrical face surfaces, known to this artas a "glove finger" presenting an open surface.

When the microporous electroconducting composite in accordance with theinvention is used for the electrolysis of alkaline halides, specificallysodium chloride, the microporous electroconducting substrate may beassociated with a metallic support constituting an elementary cathode.This is typically designated the "cathode element" or the bulkelectrode. These terms encompass a rigid metallic support whosefunction, solely, is to convey electric current, and a microporouselectroconducting material which serves as the cathode.

The microporous electroconducting material/metallic support compositemay be produced by a variety of methods. The first technique is to firstproduce the coating as described in steps (a) to (c) of the preparativeprocess for the microporous electroconducting material according to thepresent invention, then to apply the coating to the metallic supportwhich constitutes the elementary cathode, followed by sintering of thecomposite. In a preferred embodiment of the invention, the suspensiondescribed in step (a) of the process for the preparation of themicroporous electroconducting substrate is filtered directly through themetallic support constituting the elementary cathode.

The process of the invention is thus particularly preferred for theproduction of a cathode element. The cathode element thus comprises arigid electroconducting structure, such as a porous metallic support,onto which the microporous electroconducting material of the inventionis deposited.

When the subject cathode is employed in alkali metal halide electrolyticcells, or more specifically sodium chloride cells, the cathode elementmay be associated or combined with a diaphragm or membrane which servesas a separator between the cathode and anode in the electrolytic cell.

The membrane may be selected from the numerous electrolysis membranesdescribed in the literature, patent and otherwise. The cathode of theinvention constitutes an excellent mechanical support and ensures goodcurrent distribution. This current distribution is a result of theunique structure of such cathode.

The cathode element may also be in combination with a diaphragm.

The diaphragm is comprised of fibers which are microconsolidated into asheet. Such diaphragm, which may also be selected from among numerousknown electrolysis diaphragms, may also be manufactured separately. Itmay also be formed directly onto the coating of fibers defining themicroporous electroconducting material.

Those diaphragms described in EP-A-0,412,916 and EP-A-0,412,917,assigned to the assignee hereof, are the preferred. Advantageously, theasbestos-free diaphragm described in EP-A-0,412,917 is employed.

It will be appreciated that the subject composite is in certain respectsa stacked array of three layers, i.e., of the porous metallic support,the microporous electoconducting material and the membrane or diaphragm,such stacked array comprising an integral composite.

The various vacuum programs or profiles described above may be carriedout continuously or incrementally, from atmospheric pressure to thefinal pressure of about 0.01 to 0.5 bars absolute.

The sintering is typically carried out at a temperature above themelting point or softening of the fluorinated polymers which serve asbinding agents for the fibers. Sintering consolidates the coatingdescribed above.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, as in the above description, all parts andpercentages are given by weight, unless otherwise indicated.

EXAMPLE 1 (COMPARATIVE) Production of a Microporous ElectroconductingSubstrate Containing Asbestos Fibers

A suspension was prepared from the following:

(i) deionized water, the amount being calculated to provide about 4liters of suspension having a dry solids content of about 4.8% byweight,

(ii) 30 g of chrysotile asbestos fibers having an average length of 1 to5 mm and diameter of about 200 Angstrom,

(iii) 2.1 g of xanthan gum.

After stirring for several minutes, the following materials were added:

(iv) 35 g of polytetrafluoroethylene latex having a 60% dry solidscontent,

(v) 100 g of precipitated silica in the form of particles having anaverage granulometry of 3 mm and a BET specific surface area of 250 m²g⁻¹,

(vi) 70 g of carbon fibers having a diameter of about 1.5 mm and anaverage length of 10 mm,

(vii) 3.3 g of Triton X 100® marketed by Roban & Haas,

(viii) 121 g of Raney nickel in the form of a 10 mm powder (Ni 20marketed by Procatalyse).

After stirring, the suspension was deposited, by predeterminedprogrammed vacuum filtration, onto a woven iron screen laminated with"Ghent" steel having a 2 mm mesh and a wire diameter of 1 mm, theeffective surface area being 1.21 dm².

Suction was then commenced, and the pressure was decreased at 50 mbarsper minute until it reached a value of about 800 mbars. This maximumpartial vacuum was maintained for about 15 minutes.

The composite assembly was then dried and consolidated by fusing thefluorinated polymer.

The silica was eliminated "in situ" in the electrolytic cell bydissolving it in an alkaline medium, particularly during the first hoursof electrolysis.

The deposited suspension had the following characteristics:

(a) the drainage time was 200-300 s;

(b) the deposit ratio was about 70%;

(c) the final vacuum was 200 to 300 mbar;

(d) the thickness was 1.5 to 2.5 mm;

(e) the electrical resistivity was 0.5 to 2Ω cm.

The drainage time corresponded to the period of time required to filterall of the suspension (since the vacuum had been programmed; thedrainage time was a characterizing parameter of the deposition whichdepended on the filtered suspension).

The maximum partial vacuum attained by the system was maintained forabout 15 minutes, corresponding to a drying phase for the coatingformed. At the end of this drying phase, the partial vacuum was at astationary value, designated the final vacuum. The final vacuum was alsoa characterizing parameter of the deposition and dependent on thefiltered suspension. More particularly, it is a characteristic of theporous structure of the deposited coating.

The deposit ratio was determined simply by weighing the materials.

After drying or consolidation, the coating formed could be detached fromthe screen on which it had been deposited. The electrical resistivitywas measured by means of an ohmmeter connected to two conductingmetallic plates applied on the opposite face surfaces of the coating.

Production of a Composite of a Cathode Element Containing Asbestos andan Asbestos-free Diaphragm

A coating was deposited onto a cathode (microporous material+screen) toprovide a diaphragm, under the conditions described in the examples ofEP-A-0,412,917, assigned to the assignee hereof.

The initial suspension for the preparation of the diaphragm was asfollows:

(i) 100 g of polytetrafluoroethylene fibers (PTFE), introduced as 200 gof a mixture of sodium chloride and PTFE fibers (50/50 by weight) whichhad been pretreated as described below,

(ii) 60 g of potassium titanate fibers having a diameter of 0.2 to 0.5mm and length of 10 to 20 mm,

(iii) 40 g of polytetrafluoroethylene latex, containing about 65% byweight of dry solids,

(iv) 50 g of precipitated silica in the form of particles having anaverage granulometry of 3 mm and a BET specific surface area of 250 m²/g,

(v) 3.6 g of Triton X 100® marketed by Rohm & Haas.

The NaCl impregnated PTFE fibers were pretreated by mixing a solution of1 liter of water comprising about 100 g of a mixture containing about50% of PTFE fibers and 50% of sodium chloride. This operation wasrepeated, if necessary, to provide the desired amount of PTFE fibers.

The performance of the composite thus produced was evaluated in anelectrolytic cell having the following characteristics and operatingconditions:

(1) Expanded laminated titanium anode coated with TiO₂ -RuO₂ ;

(2) Woven and laminated mild steel cathode; 2 mm wires, 2 mm mesh coatedwith a microporous electroconducting material and a diaphragm;

(3) Anode-composite distance of 6 mm;

(4) Effective surface area of electrolyzer of 0.5 dm² ;

(5) Filter press cell assembly;

(6) Current density of 25 A dm⁻² ;

(7) Temperature of 85° C.;

(8) Operating with anodic chloride constant at 4.8 mole 1⁻¹ ;

(9) Electrolytic caustic soda concentration of 120 or 200 g/l.

The particular conditions employed and the results obtained are reportedin Table (III) below:

(a) RF: Faraday yield; (b) ΔU: tension at electrolytic cell terminals atspecified current density;

(c) Performance (kwh/TCl₂) is the energy consumption of the system inkilowatt hours per tonne of chlorine produced;

(d) ΔU_(I)→O is the electrolyzing tension at 85° C. and I=O byextrapolation of U=f(x) curve.

EXAMPLE 2 (COMPARATIVE) Production of Carbon Fiber-based MicroporousElectroconducting Substrate

A microporous electroconducting substrate was prepared in the samemanner as that described in comparative Example 1, from a suspensioncomprising the following:

(i) 7,000 g of deionized water,

(ii) 100 g of graphite fibers having a length of 1 to 2 mm,

(iii) 1 g of Na dioctylsulfosuccinate,

(iv) 100 g of polytetrafluoroethylene latex having about 65% hy weightof dry solids,

(v) 100 g of precipitated silica in the form of particles having anaverage granulometry of 3 mm and a BET specific surface area of 250 m²g⁻¹,

(vi) 235 g of Raney alloy.

The deposition characteristics were as follows:

(a) the drainage time was less than 100 s,

(b) the deposit ratio was about 80%,

(c) the final vacuum was less than 200 mbar.

The drainage time and final vacuum values were low. This was interpretedas being due to the formation of a microporous electroconductingsubstrate which was too thick and highly nonuniform over the entiresurface.

EXAMPLES 3-8 (COMPARATIVE) Production of an Inert Mineral Fiber-free andan Asbestos Fiber-free Microporous Electroconducting Substrate

A microporous electroconducting substrate was prepared in the samemanner as that of comparative Example 1, from a suspension comprising:

(i) 30 g of PTFE fibers introduced in the form of a mixture of sodiumchloride and PTFE fibers (50/50 by weight) which had been pretreated asdescribed above,

(ii) 70 g of carbon fibers having a diameter of about 1.5 mm and averagelength of 10 mm,

(iii) 15 or 35 g of polytetrafluoroethylene latex having about 65% byweight of dry solids,

(iv) 50 or 100 g of precipitated silica in the form of particles havingan average granulometry of 3 mm and a BET specific surface area of 250m² g⁻¹,

(v) Raney nickel,

(vi) 9 to 16 g of xanthan gum (Rhodopol® 23 marketed by Rhone-Poulenc)corresponding, respectively, to 0.06 or 0.10 weight % in water,

(vii) 0 or 3.6 g of Triton X 100® marketed by Robfin & Haas.

Deposition characteristics of the suspension are reported in Table (I):

                                      TABLE I                                     __________________________________________________________________________                           Deposition                                             Composition of suspension                                                                            conditions                                                xan-                                                                             PTFE                                                                              sil-                                                                             Tri-                                                                             Raney  deposit                                                                              V  Characteristics                                 than                                                                             latex                                                                             ica                                                                              ton                                                                              Ni  wt ratio                                                                             T  m- E   Q                                        Ex.                                                                              g  g   g  g  g   g  %   s  bar                                                                              mm  Ω cm                               __________________________________________________________________________    3  9  15  50 0  5.7 300                                                                              62  215                                                                              60 2.5 1                                        4  9  55  50 0  5.7 302                                                                              81  160                                                                              83 3   0.5-1                                    5  9  55  100                                                                              3.3                                                                              5.7 301                                                                              40  150                                                                              58 2   0.5-1                                    6  16 35  50 0  0   300                                                                              72  175                                                                              70 2   0.5-1                                    7  16 35  100                                                                              0  0   301                                                                              46  175                                                                              75 2   0.5-1                                    8  16 35  100                                                                              3.3                                                                              0   301                                                                              42  175                                                                              85 1   0.5-1                                    __________________________________________________________________________

In this and the following Tables, T represents the drainage time, Vrepresents the final vacuum (see comparative Example 1), wt representsthe weight deposited on the screen, E represents the deposit thicknessand Q represents the resistivity.

EXAMPLES 9-13 Production of a Microporous Electroconducting Subtrate inAccordance With the Invention

A microporous electroconducting substrate was prepared as in comparativeExample 1 from a suspension comprising the following:

(i) 5, 10, 50 or 100 g of potassium titanate fibers having a diameter of0.02 to 1.5 mm and length of 10 to 20 mm,

(ii) 30 g of PTFE fibers introduced in the form of a minute of sodiumchloride and PTFE fibers (50/50 by weight) which had been pretreated asdescribed above,

(iii) 70 g of carbon fibers having a diameter of about 1.5 mm an averagelength of 10 mm,

(iv) 15 or 35 g of polytetrafluoroethylene latex having about 65% byweight of dry solids,

(v) 50 or 100 g of precipitated silica in the form of particles havingan average granulometry of 3 mm and a BET specific surface area of 250m² g⁻¹,

(vi) Raney nickel containing about 70% of dry solids,

(vii) 9 g of xanthan gum (Rhodopol® 23 marketed by Rhone-Poulenc)corresponding to 0.06 weight % in water,

(viii) 2 g of Triton X 100® marketed by Rohm & Haas.

Deposition characteristics of the suspension are reported in thefollowing Table (II):

                                      TABLE II                                    __________________________________________________________________________    Composition of suspension                                                                            Deposition                                                titan               conditions                                             ate   PTFE                                                                              sil-                                                                             Tri-                                                                             Raney  deposit                                                                              V  Characterisitcs                                 fiber                                                                            latex                                                                             ica                                                                              ton                                                                              Ni  wt ratio                                                                             T  m- E   Q                                        Ex.                                                                              g  g   g  g  g   g  %   s  bar                                                                              mm  Ω cm                               __________________________________________________________________________    9  5  15  50 2  6.3 300                                                                              55  220                                                                              100                                                                              2.5 7                                        10 10 15  50 2  6.3 302                                                                              52  215                                                                              80 2.5 9                                        11 10 15  50 2  6.3 300                                                                              60  195                                                                              100                                                                              2.5 13                                       12 50 35  100                                                                              2  3   330                                                                              51  220                                                                              200                                                                              1.4 0.5-2                                    13 100                                                                              35  100                                                                              2  2.6 330                                                                              37  220                                                                              220                                                                              1.4 6                                        __________________________________________________________________________

These examples evidenced that addition of inert inorganic fibers such astitanate fibers into the microporous materials is necessary for theproduction of an industrially viable deposit. The drainage times andfinal vacuums obtained reached those measured during manufacture ofsubstrates containing asbestos fibers described in comparative Example 1(aside from Examples 3 to 8 where the drainage time remainedinsufficient and the final vacuum too low).

EXAMPLE 14 (COMPARATIVE) Production of a Xanthan-free MicroporousElectroconducting Substrate

A microporous electroconducting substrate was prepared as in comparativeExample 1 from a suspension comprising the following:

(i) 50 g of potassium titanate fibers having a diameter of 0.02 to 1.5mm and a length of 10 to 20 mm,

(ii) 30 g of PTFE fibers introduced in the form of a mixture of sodiumchloride and PTFE fibers (50/50 by weight) which had been pretreated asdescribed above,

(iii) 70 g of carbon fibers having a diameter of about 1.5 mm an averagelength of 10 mm,

(iv) 35 g of polytetrafluoroethylene latex having about 65% by weight ofdry solids,

(v) 100 g of precipitated silica in the form of particles having anaverage granulometry of 3 mm and a BET specific surface area of 250 m²g⁻¹,

(vi) 3 g of Raney nickel containing about 70% of dry solids.

Deposition characteristics of the suspension were as follows:

(a) the filtered suspension weighed 330 g,

(b) the drainage time was 120 s;

(c) the deposit ratio was about 50%;

(d) the final vacuum was 163 mbar;

(e) the thickness was 1.3 to 1.5 mm;

(f) the electrical resistivity was 10Ω cm.

EXAMPLES 15 AND 16 Production of a Microporous ElectroconductingSubstrate in Accordance with the Invention

The procedure of Example 12 was repeated, except for the amount of Raneynickel used. An amount of 5 g (Example 15) or 2.5 g (Example 16) ofRaney nickel were introduced into the suspension. The microporouselectroconducting substrate, together with the screen, constituted anasbestos-free cathode in accordance with the invention.

Production of a Composite of Asbestos-free Cathode and Asbestos-freeDiaphragm Therefor

The operating procedure of comparative Example 1 was repeated to producethis composite.

Performances and combinations are reported in the following Table (III).

It will be seen that the combination in accordance with the inventionperformed satisfactorily and had low hydrogen levels in the chlorine.These performances were comparable to a combination of a cathode elementand a diaphragm containing asbestos (comparative Example 1).

                                      TABLE III                                   __________________________________________________________________________    Microporous                                                                   material  Diaphragm                                                                           Electrolysis performances                                        wt     wt    NaOH                                                                              RF ΔU                                                                         ΔU.sub.1-0                                                                   kwh/                                                                              H.sub.2 (%) in                             Ex.                                                                              kg/m.sup.2                                                                           kg/m.sup.2                                                                          g/l %  Volt                                                                             Volt TCl.sub.2                                                                         chlorine                                   __________________________________________________________________________    15 0.28   1.95  120 98.5                                                                             3.05                                                                             2.19-2.20                                                                          2340                                                                              <0.2                                                       140 96 3.05                                                                             2.19-2.20                                                                          2400                                                                              <0.2                                                       160 92 3.05                                                                             2.19-2.20                                                                          2500                                                                              <0.2                                                       180 88 3.05                                                                             2.19-2.20                                                                          2620                                                                              <0.2                                                       200 86 3.05                                                                             2.19-2.20                                                                          2680                                                                              <0.2                                       16 0.34   1.97  120 98 3.2                                                                              2.20-2.22                                                                          2465                                                                              <0.2                                                       140 95 3.2                                                                              2.20-2.22                                                                          2540                                                                              <0.2                                                       160 92 3.2                                                                              2.20-2.22                                                                          2625                                                                              <0.2                                                       180 88 3.2                                                                              2.20-2.22                                                                          2745                                                                              <0.2                                                       200 86 3.2                                                                              2.20-2.22                                                                          2810                                                                              <0.2                                       1  0.35   2     120 98.5                                                                             3.05                                                                             2.19-2.20                                                                          2340                                                                              <0.2                                                       140 96 3.05                                                                             2.19-2.20                                                                          2400                                                                              <0.2                                                       160 92 3.05                                                                             2.19-2.20                                                                          2500                                                                              <0.2                                                       180 88 3.05                                                                             2.19-2.20                                                                          2620                                                                              <0.2                                                       200 86 3.05                                                                             2.19-2.20                                                                          2680                                                                              <0.2                                       __________________________________________________________________________

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for the preparation of an asbestos-freemicroporous electroconductive substrate, comprising (a) providing anaqueous suspension of a mixture of carbon or graphite fibers,polytetrafluoroethylene fibers, inert mineral fibers, at least onefluorinated polymer, a silica porogen, and, optionally, at least onethickening agent; (b) depositing a coating onto a porous support byprogrammedly vacuum filtering said suspension therethrough, the coatingcomprising a liquid medium; (c) removing the liquid medium from saidcoating and then drying same; (d) sintering the coating thus formed; and(e) extracting the silica porogen therefrom to form a substratecomprising intimate admixture of carbon or graphite fibers,polytetrafluoroethylene fibers and 10 to 100 parts by weight of inertmineral fibers, and optionally, at least one thickening agent,consolidated by 10 to 60 parts by weight of at least one fluorinatedpolymer.
 2. The process as defined by claim 1, said aqueous suspensioncomprising from about 2% to 5% by weight of dry solids.
 3. The processas defined by claim 1, said aqueous suspension further comprising atleast one surfactant.
 4. The process as defined by claim 1, said atleast one fluorinated polymer comprising a powder having a granulometryranging from 0.1 to 5 micrometers.
 5. The process as defined by claim 1,said porous support comprising a metal gauze or screen.
 6. The processas defined by claim 1, wherein the carbon or graphite fibers and thepolytetrafluoroethylene fibers are present in the admixture in acombined amount of 100 parts by weight.
 7. The process as defined byclaim 6, wherein the carbon or graphite fibers andpolytetrafluoroethylene fibers are present in the suspension in acombined amount of 100 parts dry weight, the inert mineral fibers arepresent in the suspension in an amount of 10 to 100 parts dry weight andthe at least one fluorinated polymer is present in the suspension in anamount of 10 to 60 parts dry weight.